The invention relates to marine propulsion systems including an internal combustion engine driving a voltage generator, e.g. a permanent magnet or wound field alternator, generating output voltage which is sensed by a voltage regulator and charges a marine battery and which supplies voltage to ignition circuitry and other boat systems providing spark ignition for running the engine.
In the prior art, overvoltage protection is provided in the regulator by turning on a pair of auxiliary SCR's to shunt output current. Overvoltage protection for the ignition circuitry is provided by a limiting zener diode. A problem in the prior art is that the setting of the threshold overvoltage for the two types of protection are at cross-purposes. On the one hand, it is desired to set the threshold overvoltage for the ignition circuitry higher than the threshold for the regulator, in order to protect the zener diode, because the zener diode cannot protect against relatively long powerful transients such as produced by permanent magnet alternator charging systems in a marine engine. On the other hand, it is desired to set the threshold overvoltage for the regulator higher than the threshold for the ignition circuitry, in order to minimize repetitive use of the auxiliary shunting SCR's, to in turn prevent overheating and failure thereof.
The present invention addresses and solves the noted trade-off problem, and achieves both of the previously incompatible desirable results, and does so with existing circuitry already present in the system.
Microprocessor based ignition control systems typically include a voltage sensor sensing the magnitude of the alternator output voltage, either directly at the alternator or at another point in the circuit and as affected by various voltage drops in the circuit, which voltage is known as the system voltage. The ignition control system adjusts dwell according to the sensed system voltage during normal engine operation to maintain substantially constant energy spark ignition. In preferred form, the present invention uses the existing voltage sensor and determines whether the system voltage is above a given threshold V.sub.T, and if not, continuing normal engine operation with normal dwell adjustment, and if so, altering the dwell to full on or full off or otherwise terminating spark ignition, to reduce engine speed and hence reduce the system voltage generated by the alternator. Sensing of the system voltage is continued, and engine speed is reduced until the voltage drops below the given threshold V.sub.T. The threshold V.sub.T is selected to be lower than the overvoltage threshold of the noted zener diode protecting the ignition circuitry and is also selected to be lower than the overvoltage threshold of the regulator to protect the noted auxiliary shunting SCR's.
The invention also provides protection in wound field alternator applications. Wound field alternators typically include transistor circuitry in the regulator which is subject to failure in a short circuit condition, and also include brushes which are subject to mechanical connection shorting to ground, both of which can cause full output current and an overvoltage condition. The present invention provides protection against both of these and other failure modes of wound field alternators by sensing the system voltage and reducing engine speed until the voltage drops below the given threshold V.sub.T.